Nanoparticles are ubiquitous and by far the most abundant particle-like entities found in natural environments and are widespread across many applications associated with human activities. There are many types of naturally occurring nanoparticles and man-made (engineered) nanoparticles. Nanoparticles occur in air, aquatic environments, rain water, drinking water, bio-fluids, pharmaceuticals, drug delivery and therapeutic products, and a broad range of many industrial products. Nanoparticles usually occur within polydisperse assemblages which are characterized by co-occurrence of differently-sized particles.
Given the widespread use of nanoparticles, the ability to control and accurately characterize their properties may be useful to many applications. Conventional methods for measuring nanoparticle properties include Nanoparticle Tracking Analysis, which uses a microscope and video camera to analyze frames of the recorded videos to track images of light reflected or scattered by the nanoparticles undergoing Brownian motion. The instrument to perform such analysis is usually comprised of a small cell, or cuvette, that enables illumination of a liquid with a very precisely defined, narrow light sheet and observation of scattered light from the nanoparticles, usually at a 90-degree angle to the light sheet. Hence the cuvette must contain at least two surfaces with minimal light attenuation properties (for example optical glass). Such cuvettes are widely used in all types of optical measurements in various laboratory instruments, are easily available and have standardized internal dimensions (in the case of the prototype 10 mm×10 mm×45 mm).
Ideally there should be no bulk movement of the liquid when the videos are being recorded so the only observed particle motion is Brownian motion. However, due to the low thermal conductivity of glass and because of potentially considerable energy transmitted from the illuminating beam to the liquid and wall of cuvette by absorption, one can observe thermally generated micro flow of the liquid regardless of the volume of liquid in a traditional cuvette. Other sources of micro flows are possible, for example movements of the table on which the instrument is mounted that cause vibrations of the table or evaporation of the sample liquid that cools its surface. Flow can also be induced by stirring the liquid in the cuvette, or by pumping liquids in and out of the cuvette. In these and other induced flow cases, it is desirable to arrest the flow as quickly as possible for effective and timely particle analysis. Algorithms are available to detect and remove effects of such bulk liquid movement; however, these algorithms have limitations and more accurate results are achieved in the absence of bulk liquid movement.
Another desirable situation for optimal detection and processing of scattered light from nanoparticles in liquids is to minimize or eliminate backscattering of light from the wall of the cuvette that is opposite to the wall where light enters the cuvette (the back wall). Such backscattering of the incoming light beam typically broadens the illuminated region (thickening of light sheet), thus creating images that could be partially out of focus of the microscope (fuzzy images), which are not suitable for precise particle tracking. Backscattering induced broadening has an inherently inconsistent impact on the width of the light sheet and as such also causes variability in particle concentration measurements since the width of the light sheet effects the volume of sample that is being analyzed in each measurement. Secondarily deleterious light scattering effects from other reflective surfaces in the cuvette should also be minimized through use of light absorbing materials or coatings (such as black paint).
Another important consideration is compatibility with existing components that accurately hold the cuvette in place relative to the light sheet, control its temperature and enable stirring and or pumping of the liquid. Such stirring and/or pumping facilitates examination of multiple fresh aliquots from the same sample within the cuvette and is easily achieved with a magnetic stirring bar at the bottom of the cuvette which is driven by an external rotating magnet, or with an external pump.
What is needed, therefore, is an improved system that can minimize movement of the liquid while also eliminating backscatter of the light within the observation region of the cuvette.